1. Field of the Invention
This invention relates to a method for making an electrowetting device, more particularly to a method for making an electrowetting device with a hydrophobic coating layer.
2. Description of the Related Art
An electrowetting device typically includes a liquid encapsulated therein which can change in surface electric properties when an external voltage is applied to the liquid to control operation modes of the liquid. The electrowetting device may be applied to a display, an optical liquid lens, a biochip, etc.
Referring to FIG. 1, a conventional electrowetting device comprises a substrate 11, a hydrophobic layer 14 formed on an upper surface 111 of the substrate 11, a surrounding wall 15 formed on the hydrophobic layer 14, a microchamber 16 surrounded by an inner surface of the surrounding wall 15 and formed over the hydrophobic layer 14, a top electrode 13 disposed above the surrounding wall 15, and a liquid 17 disposed in the microchamber 16 and on the hydrophobic layer 14 and disposed between the substrate 11 and the top electrode 13. The substrate 11 includes a base plate 112, a conductive layer 113, and an insulating layer 114. The liquid 17 includes first and second solutions 171, 172 that are immiscible with each other.
The top electrode 13 is an indium tin oxide (ITO) conductive glass.
By applying a voltage between the conductive layer 113 and the top electrode 13, the surface electric properties of the hydrophobic layer 14 and the liquid 17 are varied.
For example, if the electrowetting device is applied to a display, the first solution 171 may be a colored ink, and the second solution 172 may be water. When no external voltage is applied, the first solution 171 forms a rounded shaped liquid drop on the hydrophobic layer 14 as shown in FIG. 1. When an external voltage is applied, the surface electric properties of the first solution 171 and the hydrophobic layer 14 are varied (i.e., an interfacial tension between the first solution 171 and the hydrophobic layer 14 is varied). Accordingly, the first solution 171 moves aside, as shown in FIG. 2.
A method for producing the conventional electrowetting device comprises: (1) preparing the substrate 11; (2) forming the hydrophobic layer 14 on the upper surface 111 of the substrate 11; (3) forming the surrounding wall 15 on the hydrophobic layer 14 to define the microchamber 16; and (4) disposing the liquid 17 into the microchamber 16.
However, since the hydrophobic layer 14 is formed from a fluorine-containing polymer or a derivative thereof (for example, polytetrafluoroethylene, or a derivative of Polytetrafluoroethylene), and since the surface tension of the fluorine-containing polymer or the derivative is extremely small, an adhesion between the hydrophobic layer 14 and the surrounding wall 15 is relatively poor.
In order to improve the adhesion, before forming the surrounding wall 15, the upper surface of the hydrophobic layer 14 is modified by plasma treatment, irradiating with UV light, dipping in a chemical solvent, etc., to have a temporary or permanent modified surface. Accordingly, the surrounding wall 15 may be readily adhered to the modified surface of the hydrophobic layer 14.
However, surface modification of the hydrophobic layer 14 requires a relatively complicated process which increases the cost for producing the electrowetting device. Furthermore, the electrowetting device may encounter problems with respect to the reliability thereof due to the modified surface of the hydrophobic layer 14. On the other hand, when the electrowetting device is applied to a display, because the liquid drop of the first solution 171 is thick at its middle portion and thin at its lateral portion as shown in FIG. 1, leakage of light can occur at the lateral portion of the liquid drop, which is adverse to a contrast ratio of the electrowetting device in the display.